Use absolute path for temp directory.

Before this change, the following would fail:

```javascript
var fs = require('fs')
var quickTemp = require('quick-temp')

var somePathObj = {}
fs.mkdirSync('somePath')
process.chdir('somePath')
quickTemp.makeOrRemake(somePathObj, 'tmpDestDir')

var someOtherPathObj = {}
fs.mkdirSync('someOtherPath')
process.chdir('someOtherPath')
quickTemp.makeOrRemake(someOtherPathObj, 'tmpDestDir')
```

This fails because the local `baseDir` var is set/cached with a value
that references `tmp`, but once `process.cwd` changes we don't check for
the existence of `tmp/` again.
Merge pull request #3 from rjackson/check-baseDirRoot

Use absolute path for temp directory.